Mask ROMs of semiconductor devices have nonvolatile characteristics in that data is retained in the memory even if the power supply is turned off. In addition, only read operations with respect to already-written data can be performed with the mask ROMs. Because the mask ROMs are coded with the data in respective cells during the process of forming the mask ROM devices, the data stored in the mask ROMs cannot be erased or rewritten. Generally, a coding method selectively implants impurity ions after forming cells constituted with MOS transistors.
FIGS. 1 and 2 are cross-sectional views illustrating a conventional coding method of a mask ROM device.
With reference to FIGS. 1 and 2, a device isolation layer (not shown) is formed on a semiconductor substrate 1 having first and second regions 10 and 20 to define respective active regions. The first region 10 is a region in which cells not performing a program process are formed, and the second region 20 is a region in which cells performing the program process are formed. N-type impurity ions are implanted on the surface of the active region to form a surface doped layer 2.
A gate oxide layer 3 and a gate electrode 4, which are sequentially stacked, are formed on the active region. A plurality of gate electrodes 4 are formed on the substrate 1. N-type impurity ions are implanted using the gate electrode 4 as a mask to form source/drain regions 5. The surface doped layer 2 under the gate electrode 4 corresponds to a depletion mode channel region. The gate electrode 4, the source/drain regions 5, and the depletion mode channel region form a unit cell of a mask ROM device. By the depletion channel region, the cells are in a turn-on state.
A lithography process is performed on the semiconductor substrate 1 to form a photoresist pattern 6. An opening 7 for exposing a selected cell can be formed in the photoresist pattern 6. In other words, the photoresist pattern 6 covers the cell in the first region 10, and the opening 7 exposes the cell in the second region 20. The opening 7 exposes a gate electrode 4 of the selected cell. In addition, the opening 7 may expose a part of the source/drain regions 4 of the selected cell.
P-type impurities are implanted using the photoresist pattern 6 as a mask to dope P-type impurities in the depletion mode channel region of the selected cell. Thus, the selected cell can be changed to a turn-on or turn-off state according to the voltages applied to the gate electrode 4.
In the above-mentioned conventional method for coding the mask ROM device, P-type impurity ions for programming penetrate the gate electrode 4 of the selected cell to be implanted into the depletion mode channel region. Accordingly, lattice damages may occur in the gate electrode 4 of the selected cell by ion implantation. Additionally, there may be intrusions at the interfaces between the gate electrode 4 and the gate oxide layer 3 and/or the gate oxide layer 3 and the active region. Therefore, the characteristics of leakage current in the selected cell may degrade. In addition, the P-type impurity ions are implanted in a Gaussian profile and may be implanted into the source/drain regions 5 of the selected cell. Accordingly, different types of impurities are implanted into the source/drain regions 5 of the selected cell, and thereby degrading punch-through characteristics between the source/drain regions 5 of the selected cell.
Furthermore, high ion implantation energy is required because P-type ion impurities may penetrate the gate electrode 4. Therefore, high-energy ion implantation apparatus is needed, and it results in poor productivity.